Hitherto, adjustment of the rate of airflow blown for a purpose of air-cooling/heating of the car and the like was realized by the use of control means for switching in a step-by-step fashion a voltage or current being applied to a blower motor or of another control means for controlling continuously the same through transistors and the like.
Accordingly, in the case of switching in the step-by-step fashion there were the problems that the flow rate can be adjusted merely step-by-step, e.g., switched among four or five steps and, because a switch for large current is employed, its operation load is relatively heavy resulting in severe operation characteristics. Further, in the case of varying continuously the voltage and the like for the blower motor, there were the problems that the control becomes unstable, as will be described hereinafter, when the flow rate is small, for example, when the motor is started at a slow speed and, when the flow rate is maximum, i.e., when the motor is to be operated at the maximum speed. sufficient voltage can not be applied to the motor owing to the internal voltage drop of a voltage-controlling transistor, thus, a sufficient flow rate can not be obtained in the morning of the severe cold season and the like when the voltage of a battery is apt to drop.
More clearly, the prior art will be described with reference to FIGS. 1 through 4 illustrating examples of the conventional motor drive control device.
In these drawings, 1 is a blower motor, 2 is a flow rate change switch, and TR1 is a transistor.
In FIGS. 1, the blower motor 1 is one for supplying the air at the time of air-heating/cooling of the car and the like. Voltage Vm to be applied to the blower motor 1 is the battery voltage Vb minus the voltage drop determined by a resistance of a resistor connected to a terminal which is selected by the flow rate change switch 2. Accordingly, by changing arbitrarily the setting position of the flow rate change switch 2, the voltage Vm to be applied to the blower motor 1 can be changed to vary selectively the flow rate.
FIG. 2 illustrates the variation of voltage Vm to be applied to the blower motor 1 in correspondence to the setting positions of the flow rate change switch 2. For example, at SW (switch) position "OFF" the voltage Vm is zero volt, at SW position "LO" the voltage Vm is "VO", and so on. In this way, by changing the SW position from "OFF" to "LO", "M1", "M2" and "H1", the voltage Vm applied to the blower motor 1 is varied in the step-by-step fashion thereby resulting in a variation in flow rate. To achieve the foregoing operation, the flow rate change switch 2 must be operated which has a contact area and a contact pressure sufficient for direct switching of the current supplied to the blower motor 1. As a result, the prior art has the drawback that the operation load of the flow rate change switch 2 is relatively heavy and its operation characteristic is bad.
FIG. 3 illustrates another example where a transistor TR1 is employed to control the voltage Vm to be applied to the blower motor 1.
In this system, because the voltage Vm applied to the blower motor 1 is varied by the use of the transistor TR1, there is no need of operating the flow rate change switch 2 shown in FIG. 1 of bad operability.
However, as shown in FIG. 4, when the transistorcontrolled blower motor 1 is started by increasing progressively an RV stroke of a variable resistor for continuous change of the flow rate to apply a voltage gradually to the blower motor 1, there appears an intermittent characteristic of voltage as indicated by the mark .circle.1 . Further, when the RV stroke is increased to the maximum, a voltage drop Vce appears on the transistor TR1, thus, the maximum voltage applied to the blower motor 1 becomes equal to (Vb-Vce) which is a source voltage Vb minus the voltage drop Vce. That is, this system also has the drawback that the efficiency is low and the blower motor 1 can not provide the maximum speed resulting in a shortage of flow rate.